The invention relates to neurostimulation of a subject by an external electric field, induced by field electrodes positioned away from the subject. Fluctuations of the field induce electric currents in the subject's body, since bulk biological tissue is a rather good conductor of electricity.
A neurological effect of external electric fields has been mentioned by Norbert Wiener, in discussing the bunching of brain waves through nonlinear interactions. The electric field was arranged to provide "a direct driving of the brain". Wiener describes the field as set up by a 10 Hz alternating voltage of 400 V applied in a room between ceiling and ground.
In U.S. Pat. No. 5,169,380 Brennan describes an apparatus for alleviating disruptions in the circadian rythms of a mammal, where an alternating electric field is applied across the head. The voltage applied to the electrodes is specified as at least 100 V, and the peak-to-peak value of the field as at least 590 V/m in free air before deploying the electrodes across the head of the subject. The alternating electric field has a frequency in the range from 5 to 40 Hz. Brennan states that the method is aimed at subjecting at least part of the subject's brain to an alternating electric field, in the belief that this would stimulate the influx of calcium ions into nerve endings, which in turn would "regulate and facilitate the release of neurotransmitters". It should be noted that electric polarization of the head causes the field strength in the narrow space between electrode and skin to be about a factor h/2d larger than the free-air strength, h being the distance between the electrodes and d the spacing between electrode and skin. For h=17 cm and d=5 mm the factor comes to 17, so that with the specified free-air field of at least 590 V/m the field in the gap between electrode and skin is at least 10 KV/m peak to peak.
A device that involves a field electrode as well as a contact electrode is the "Graham Potentializer" mentioned by Hutchison. This relaxation device uses motion, light, and sound as well as an external alternating electric field, applied mainly to the head. The contact electrode is a metal bar in physical contact with the bare feet of the subject; the field electrode has the form of a hemispherical metal headpiece several inches from the subject's head. According to the brief description by Hutchison, a signal of less than 3 Volts at a frequency of 125 Hz is applied between the field electrode and the contact electrode. In this configuration, the contact electrode supplies to the body the current for charging the capacitor formed by the headpiece field electrode and the apposing area of skin. The resulting electric field stands mainly between the head piece and the scalp. In the three external field arrangements mentioned, viz., Wiener, Brennan, and Graham, the electric field is applied to the head, thereby subjecting the brain to polarization currents. These currents run through the brain in a broad swath, with a distribution determined by the bulk geometry and nonuniformities of conductivity and permittivity. The scale of the current density is conveniently taken as its maximum value on the skin of the head. For sinusoidal fields this scale is easily calculated as the product of radian frequency, vacuum permittivity, and maximum amplitude of the external field on the head. Using Brennan's lowest frequency of 5 Hz, his minimum free-air field strength of 590 V/m, and the factor 17 as estimated above to account for polarization of the head by the applied field, the scale of the polarization current density in the brain comes to about 280 pA/cm.sup.2. In the absence of an understanding of the neurological effects involved, it is prudent to avoid exposing the brain to artificial bulk currents of such scale, and apply a factor 1/4000 for safety. Accordingly, polarization current densities in the brain in excess of 70 pA/cm.sup.2 are considered substantial.
It is an object of the present invention to obtain a method and apparatus for manipulating the nervous system by means of external electric fields without causing substantial polarization currents in the brain.
The use of electric fields raises concerns about possible health effects. Such concerns have been widely discussed in the news media in regard to electric power lines and electric apparatus. Answering the pertinent questions by objective research will take time, but meanwhile governments have been setting guidelines for safe limits on field strengths. At present, the strictest conditions of this sort are the Swedish MPRII guidelines. Magnetic fields are of no concern here, because the currents involved are so small. But the electric field must be considered, since even at low voltages strong electric fields can result from field electrodes placed close to the skin. With respect to extremely low frequency electric fields, the MPRII guidelines limit the field strength to 25 V/m in the frequency range from 5 Hz to 2 KHz. In the Brennan patent the minimum field strength of 590 V/m violates the guidelines by a factor 24; when polarization effects are accounted for, the factor is about 400. It is a further object of the present invention to manipulate the nervous system by using external electric fields that are in compliance with the MPRII guidelines.
Brennan stipulates voltages of at least 100 V, and as high as 600 V for the preferred embodiment. Generation of such voltages requires a voltage multiplication stage, if low voltage battery operation is desired. This increases the current drain and the size of the generator. The large voltages also raise safety concerns. It is yet a further object of the present invention to manipulate the nervous system by means of external electric fields, using low voltages that are generated by small and safe battery-operated devices with low current consumption.
In the arrangements of Wiener and Brennan, the electric field is induced by field electrodes that are positioned at opposite sides of the subject. This limits portability and convenience of use. It is a further object of the present invention to provide a method and apparatus for the manipulation of the the nervous system by electric fields in a manner which does not require field electrodes placed at opposite sides of the subject's body.